Lasers producing picosecond output pulses, or “picosecond lasers,” are useful in many scientific applications. For example, a parametric laser cavity producing high-power, tunable, picosecond pulses, is useful for nonlinear optical studies of narrow-band transitions in the near- and mid-infrared spectral regions. Nd-based, solid-state lasers having an Nd-doped active medium, are the most common and widely used picosecond lasers.
Generally, picosecond lasers, such as an Nd-based, solid-state laser, are constructed in one of two ways in order to generate energetic, picosecond pulses. In a first exemplary construction, the laser contains a regenerative amplifier (RGA) for amplifying a seed pulse from about 10 μJ (micro-Joule) up to the moderate level of 1 to 10 mJ (milli-Joule). The laser also contains a power amplifier for boosting the pulse energy further up to about 100 mJ (milli-Joule). Such a laser is usually built using a hybrid system consisting of a low-power, diode-pumped, continuous-wave, mode-locked laser, and pulsed, flashlamp-pumped, regenerative power amplifiers. By combining two different laser platforms, such a laser is both expensive and complicated to use.
In a second exemplary construction, the Nd-based laser contains a pulsed Q-switched and mode-locked oscillator for generating a short pulse of 1 to 10 mJ, and a power pulsed amplifier for amplifying the pulse power to about 100 mJ. As is well known, Q-switching is a rapid change of optical losses, i.e., parameter “Q”, of an optical resonator resulting in generation of an intense pulse with short, e.g., nanosecond, duration. Mode locking is a method to produce very short pulses of picosecond or femtosecond duration. Both Q-switching and mode locking can be accomplished using active as well as passive devices. A Q-switched and mode-locked laser produces trains of picosecond pulses with the duration of each train being tens to hundreds of nanoseconds. Because both the oscillator and the amplifier are built using the same components, operation of this laser is greatly simplified.
The Nd-based pulsed picosecond laser described above requires a saturable absorber (SA) with a fast recovery time, or a “fast SA.” A fast SA typically used in pulsed picosecond lasers is a dye solution. While a dye solution operates fairly reliably, it may be toxic and must be replaced and maintained on a regular basis.
Progress has been made in the development of fast, semiconductor-based SAs for mode-locking solid-state lasers. However, while these fast, solid-state SAs perform successfully in continuous-wave lasers, they have not been reported as being operable in pulsed, solid-state lasers.
Slow, solid-state SAs can produce ultra-short pulses, provided fast gain depletion or soliton formation occurs in the laser resonator. Unfortunately, these conditions are difficult to obtain in pulsed, flashlamp-pumped Nd3+:YAG lasers.
The '027 patent describes a laser structure with a solid-state SA, which, unlike a dye solution SA, does not require replacement and maintenance over the lifetime of the laser, and does not have the problems associated with conventional solid-state SAs.
The Cr4+:YAG crystal has certain characteristics that make possible its use as a slow SA for passively Q-switching Nd-based lasers. One such characteristic of the Cr4+:YAG crystal is its strong absorption band near 1060 nm that allows it to perform as a saturable absorber at the wavelength of an Nd-based laser (1064 nm). Another of these characteristics is its absorption recovery time of about 8 μs. This recovery time makes Cr4+:YAG an ideal passive Q-switch for an Nd-based lasers. For an SA to perform well as a passive mode-locker, however, the SA must have an absorption recovery time much shorter than the round-trip cavity time (usually on the order of 5-10 nanoseconds) and similar to, or shorter than, the desired duration of a laser output pulse. Because of its relatively long absorption recovery time, the Cr4+:YAG crystal cannot passively mode-lock the laser to produce picosecond pulses.
A pulsed Nd3+:YAG picosecond laser using a fast SA dye solution typically produces output pulses of 30 to 40 picoseconds. A negative feedback technique can be used to control pulse duration and energy stability in mode-locked lasers. For example, a passive negative feedback element can be used to shorten the pulse duration of a Nd3+:YAG picosecond laser with a fast SA dye solution to 10 to 15 picoseconds.
The '027 patent describes a pulsed, solid-state, Q-switched and mode-locked laser for generating short picosecond pulses of stable energy, which does not require a fast SA dye solution, is simple to use and is very stable. An exemplary laser described therein comprises a solid-state laser medium, such as an Nd3+-doped crystal, a saturable absorber (SA) for Q-switching, and a passive negative feedback (PNF) element. The SA element is “slow,” having an absorption recovery time which is longer than a desired duration of an output pulse. Typically, this slow SA would not be capable of operating well on its own as a passive mode-locking element. In the exemplary laser of the '027 patent, however, the combined action of a slow SA and PNF elements allows the pulsed mode-locked laser to produce an output pulse or pulses of the desired duration. In an exemplary embodiment, the laser comprises a Nd3+:YAG laser medium. The slow SA is preferably a Cr4+:YAG crystal and the PNF element is preferably a GaAs wafer.
The solid-state laser (oscillator) of the '027 patent is capable of producing very short, energetic output pulses having a duration on the order of one or more picoseconds, such as from about 1 to about 200 picoseconds, and an energy of from about 100 μJ to about 2 mJ.
In addition to the aforementioned properties, it is also desirable for a picosecond laser to generate pulses whose durations can be varied. It has been observed that the pulse duration of lasers such as that of the '027 patent having an SA of Cr4+:YAG has a marked dependency on the position of the SA in the resonator.
Generally, the angular dependence of saturation fluence in the crystalline SA (such as Cr4+:YAG and LiF:F2−) causes pulse energy and duration variations in Q-switched lasers when the crystal axis is rotated relative to the polarization of the laser radiation. In the case of Cr4+:YAG, pulses with the highest energy and shortest duration are generated when the optical polarization is along the crystal axis, whereas pulses with the lowest energy and longest duration are generated when the optical polarization is at 45° to the crystal axis. The pulse parameters also depend on the ratio of the intensities (or mode sizes) in the active medium and the SA: the higher the relative intensity in the SA, the shorter, more energetic the Q-switched pulse that is generated.
Several publications describe the dependence of the output parameters of a Q-switched laser on the orientation and position within the laser resonator of the passive Q-switch, i.e., the Cr4+:YAG SA element. See, e.g., I. V. Klimov, V. B. Tsvetkov, I. A. Shcherbakov, J. Bartschke, K.-J. Boller, and R. Wallenstein, CW-diode-pumped Nd:GdVO4-laser passively Q-switched with Cr4+:YAG as saturable absorber, Proceedings of Advanced Solid State Lasers Conference, 172-174 (1996); N. N. Il'ichev, A. V. Kir'yanov, and P. P. Pashinin, Model of a passively Q-switched laser accounting nonlinear absorption anisotropy in a passive switch, Proceedings of Nonlinear Optics: Materials, Fundamentals, and Applications Topical Meeting, 113-115 (1998).